1. Field of the Invention
The present invention relates to an image reader such as an image scanner, and an image reading lens suitable for loading an original image in an image-forming apparatus such as a facsimile machine or a digital copier. The present invention particularly relates to a compact and high-performance image reading lens having the small number of lenses, an image reader using such an image reading lens, and an image-forming apparatus using such an image reader.
2. Description of the Related Art
An image reader in an image scanner, a facsimile, a digital copier, or the like is configured to reduce an image of an original to be read by an image reading lens, image the reduced optical image by an imaging element such as a CCD (charge-coupled device), and change the image information into electric image data. In order to read full-color original information, the imaging element includes a so-called three-line CCD image sensor in which light-receiving elements having red, green, and blue filters are arranged in three lines, and the reduced optical image of the original is formed on the light-receiving surface of the imaging element so as to decompose the reduced optical image into three primary colors, so that the color image information is converted into signals.
The above-described image reading lens is required to have high contrast in a high spatial frequency area on an image surface and aperture efficiency close to 100% in an angular field peripheral portion. Moreover, in order to preferably read a color original, it is necessary to align red, green, and blue imaging positions with the optical axis direction on a light-receiving surface, and the chromatic aberration of each color must be preferably corrected.
A so-called Gaussian image reading lens which is widely used as an image reading lens has a relatively large diameter which achieves a high resolution. However, in order to achieve a recently-required high performance with such a thin configuration, many lenses (i.e., six lenses or more), for example, a four-group configuration having six lenses or a five-group configuration having eight lenses are required.
On the other hand, JP 3939908B discloses an image reading lens made of three to five lenses which are a small number of lenses relative to the Gaussian image reading lens. The image reading lens has a performance which is equal to that of the Gaussian image reading lens or more.
The image reading lens described in JP 3939908B includes three lenses at minimum, and achieves an optical system which can achieve a high performance by easily correcting aberrations with the lens closest to the image side (hereinafter referred to as “the most image side lens”) having a negative power near an image surface.
As described above, JP 3939908B discloses the optical system which can achieve a high performance by easily correcting aberrations with the most image side lens having a negative power near an image surface and a small number of lenses such as three to five lenses.
However, JP 3939908B is silent about temperature rise inside the image reader and deterioration in performances associated with the temperature rise due to increase in the amount of heat generation near an imaging portion such as a CCD image sensor associated with downsizing in an image reader and increase in an image reading speed, and also increase in the amount of heat generation associated with increase in luminance of a light source illuminating an original image. JP 3939908B also does not describe the control of the deterioration in performances due to such temperature rise.
Heat expansion and a change in a refractive index of a material of an image reading lens occur due to temperature rise inside the image reader, so that a change in a focal length of an image reading lens occurs. The heat generation near the imaging portion causes heat expansion of a holder which holds the imaging portion such as a CCD image sensor and the image reading lens, deteriorating in a performance due to a gap between the position of the imaging surface and the image-forming position of the image reading lens.